A thermal barrier coating (TBC) system may be used to protect the components of a gas turbine engine that are subjected to the highest material temperatures. The TBC system usually includes a bond coat that is deposited upon a superalloy substrate, and a ceramic TBC that is deposited upon the bond coat. The TBC acts as a thermal insulator against the heat of the hot combustion gas. The bond coat bonds the TBC to the substrate and also inhibits oxidation and corrosion of the substrate.
One currently used TBC is a stabilized zirconia, which is zirconia (zirconium oxide) with an oxide added to stabilize the zirconia against phase changes that otherwise occur as the TBC is heated and cooled during fabrication and service. The stabilized zirconia is deposited by a physical vapor deposition process such as electron beam physical vapor deposition (EBPVD). In this deposition process, the grains of the stabilized zirconia form as columnar structures that extend generally outwardly from and perpendicular to the substrate and the bond coat.
To be effective, the TBC system must have a low thermal conductivity and be strongly adherent to the article to which it is bonded under contemplated use conditions. To promote adhesion and to extend the service life of a TBC system, an oxidation-resistant bond coat is usually employed. Bond coats are typically in the form of overlay coatings such as MCrAlX, where M is a transition metal such as iron, cobalt, and/or nickel, and X is yttrium or another rare earth element. Bond coats are also diffusion coatings such as simple aluminide or platinum aluminide. A notable example of a diffusion aluminide bond coat contains a platinum intermetallic, e.g. NiPtAl. When a diffusion bond coat is applied, a zone of interdiffusion forms beneath a diffusion bond coat. This zone is typically referred to as a diffusion zone.
During exposure of the ceramic TBC and subsequent exposures to high temperatures such as during ordinary service use thereof, bond coats of the type described above oxidize to form a tightly adherent alumina scale that protects the underlying structure from catastrophic oxidation.
The columnar structure of the TBC system is of particular importance to adherence of the coating and to the coating maintaining a low thermal conductivity. In addition to gaps between columns, there also exists a fine porosity within subgrains in the columnar structure. The fine porosity is sometimes observed to be oriented substantially orthogonal to the columns.
As the stabilized zirconia is cycled to elevated temperatures during service, sintering creates the problems of both the large-grain, inter-columnar porosity and the subgrain, fine porosity being gradually closed. As a result, the ability of the stabilized zirconia to accommodate thermal expansion strains gradually is reduced, and the thermal conductivity of the stabilized zirconia gradually increases by about 20 percent or more.
It has been recognized that the addition of sintering inhibitors to the stabilized zirconia reduces the tendency of the gaps between the columnar grains to close by sintering during service of the thermal barrier coating. A number of sintering inhibitors have been proposed. However, these sintering inhibitors have various shortcomings, and there is a need for more effective sintering inhibitors.
Some of the physical demands of a gas turbine blade include operation in extreme environments. One condition to which a gas turbine blade is subjected is the erosive effect of small particles that pass across the turbine blade. The small particles can be generated a part of the combustion process inside a gas turbine. Another condition that a gas turbine blade is subjected to is foreign objects that come into the gas stream.
What is needed is a TBC that avoids at least some of the problems that existed in the prior art.